The present invention relates generally to the field of instruments for spectrally measuring and analyzing optical properties of samples. Such instruments are presently used in industrial and agricultural applications for colorimetry and for quantitatively analyzing the constituents of samples. Additional applications for such instruments are being developed in the field of medicine in which samples are spectrally analyzed for diagnostic purposes.
Examples of agricultural applications presently in use are instruments which accurately determine the oil, protein and water content in grain or soy beans. The traditional analytical laboratory techniques, such as the Kjeldahl technique for measuring protein, are extremely accurate but require the services of a skilled chemist. The results, furthermore, are not immediately or readily available. Buyers of agricultural products have demonstrated an increasing interest in accurate and rapid determinations of the moisture, protein and oil percentages of the various produces purchased. The wheat export market, for example, has seen the widespread introduction of selling on the basis of guaranteed protein content. This competitive pressure has increased the requirement of the commodity handler, from the country elevator to the export terminal, to rapidly and accurately sort grains and other products by their protein percentage, as well as by oil and water content, where applicable. The need for versatile, yet low cost, advanced equipment, which combines and improves upon recent scientific findings in the field of nondestructive testing of agricultural products has greatly increased. For maximum usefulness of commodity handlers, such an instrument must not place high demands on the skillfulness of the operator or require a specialized knowledge of the scientific basis for the end result.
Recent developments have provided instruments which are able to satisfy some of the above requirements of commodity handlers. The optical analyzer described by Donald R. Webster in U.S. Pat. No. 3,861,788, assigned to the assignee of the present application, provides an automatic test instrument for gauging the percentage of various constituents in organic substances by comparing the reflective optical density of the sample at various wavelengths. This device contains narrow band optical filters connected together in the form of a rotatable paddlewheel positioned so that the filters can be individually swept through the incident light path between the specimen and a wideband light source. As the filter wheel turns, the band of light passed by each filter is progressively shifted with the changing angle of the filter relative to the light path. The filter wheel configuration includes opaque vanes extending from the ends of the filters to periodically interrupt the passage of light to the specimen. Photodetectors are positioned to sense the level of light reflected from the specimen. The output of these photodetectors is sampled at predetermined times relative to the rotation of the filter wheel to yield values indicative of reflected intensity at certain wavelengths. An electronic circuit utilizes this data to calculate three optical density difference values corresponding to moisture, protein and oil content of the specimen sample. The difference values are automatically inserted into three linear equations which are solved to obtain readings representing the three percentages of oil, water and protein content of the specimen. Each time a new specimen is loaded for testing, the instrument described in U.S. Pat. No. 3,861,788 is automatically calibrated against a standard sample, preferably Teflon (trademark). The output of the photodetectors is amplified in a special circuit which subtracts the level of dark period current from the output when the photodetectors are illuminated.
A related, but earlier, instrument is described by Eugene R. Ganssle and Donald R. Webster in U.S. Pat. No. 3,765,775, entitled "Optical Internal Quality Analyzer", and also assigned to the assignee of the present application. The specimen sample therein is illuminated with light sequentially filtered by a continuously rotating disc carrying a plurality of narrow bandwidth optical interference filters. The combined output of several photodetectors positioned to receive light transmitted through or, alternatively, reflected by the specimen is selectively sampled after passing through a logarythmic amplifier to obtain readings at two discrete wavelengths which are then compared in a differential amplifier to provide the required measurments. Although the system described in U.S. Pat. No. 3,765,775 is satisfactory for its intended purpose, its ability to make readings at various wavelengths is naturally limited by the number of filters carried by the disc. It is, therefore, not possible to take readings at wavelengths between those of two adjacent filters. This limitation is mitigated somewhat by the filter wheel construction of the aforementioned U.S. Pat. No. 3,861,788.
Yet another recent prior art photo-optical technique for determining, for example, the fat content of meat is described by George F. Button and Karl H. Norris in U.S. Pat. No. 3,877,818 owned by the United States of America. This technique, developed at the U.S.D.A. Agricultural Research Service in Greenbelt, Maryland utilizes an instrument wherein a meat sample is exposed to infrared radiation from an incandescent light source. The radiation is transmitted through or reflected from the meat sample onto a tilting mirror which causes the respective transmitted or reflected light from the meat to pass through a planar interference filter at varying angles of incidence. Varying the angle of incidence of the filter by oscillating the tiling mirror produces a corresponding change in the wavelength of the radiation passing through the filter over a narrow bandwidth in the infrared spectrum. A photodetector receives the light transmitted through the filter and generates an electrical signal that is processed to read the fat content of the sample.
These and other prior art optical analyzers are limited in the accuracy of their measurements by the particular optical components and systems utilized for spectral analysis. The variety of agricultural products requiring very accurate content analysis has increased the demand for optical instruments having greater capability for more precise measurements of constituent content.